Biomedical Engineering Reference
In-Depth Information
Thus, mechanical treatment of reactants can form metallic-ceramic
composite powder particles. The structure quality, that is whether such
composite particles are micro- or nanosized, depends on the type of reagents
and milling conditions.
12.2.2 Mechanochemical synthesis: mechanical activation,
mechanical alloying, reactive milling
Mechanochemical synthesis includes mechanical alloying together with
reactive milling which follows from mechanical activation of reagents. This
is also done by high-energy ball milling (Murty and Ranganathan, 1998;
Avvakumov et al., 2001; Suryanarayana, 2001; Suryanarayana et al., 2001;
Boldyrev, 2006; Smolyakov et al., 2007; Bala´ zˇ , 2008) which is a more
complex but very effective process of metallic-ceramic composite formation.
There are several variants of mechanochemical syntheses, which depend on
the nature of the initial reagents nature (e.g. two different metals with
ceramics, metal oxide with active metal or metal salt with active metal, and
others).
Mechanical activation is responsible for enhancing the reactivity of solids
by enlarging the surface area or accelerating the reaction by correct mixing
of the reagents. In the processes of plastic deformation and fracture and
friction during ball collisions, the impact energy is converted into other
forms of energy. These induce structural defects, broken bonds and other
forms of excess energy. These accumulate and a new, active state of the
substances is produced. Consequently, the chemical reactivity of the solids
increases significantly (e.g. Chen et al., 1997).
Mechanical alloying is the process in which mixtures of powders are
milled to achieve alloying at the atomic level. During this process, when
high-energy impulses are used, metallic powders can form alloys like
different solid solutions, intermetallic phases, mixtures of metals or
amorphic materials with properties that are very often different from
those found when using traditional methods (McCormick et al., 1989;
Maurice and Courtney, 1990; Schwarz, 1996; Urakaev and Boldyrev, 2000).
It is important to note that any form of metal alloy that comprises
composite components has better physical and mechanical properties than
pure metals. For example, consolidated materials based on intermetallics of
nickel, iron, titanium with aluminium are treated as functional materials
that have specific physical properties and as constructive materials that have
unique mechanical features such as structural stability at high temperature.
Especially important are their high melting temperature, high mechanical
resistance and low density. These properties allow them to be used in the
automobile industry (turbo compressor rotors, valves, combustion chamber
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